Switching power supplies are typically used in applications where a large and heavier linear type supply would not be appropriate. Switching power supplies typically rectify an alternating current (AC) power source where the resultant direct current (DC) voltage is fed to a group of high-frequency, high-power switching transistors. These switching transistors chop the DC voltage into a high frequency AC signal. The high frequency AC signal is then fed to a high frequency transformer that is wound to produce the correct output voltage. A high-frequency transformer is used since it can accommodate a great deal more power for its weight and size than those operating at a typical 50–60 Hz line frequency. Moreover, the filter capacitors used at the output of the high-frequency transformer can be much smaller than those operating at 60 Hz. Thus, a switching power supply can operate at a given voltage and current at a significantly smaller size than a linear power supply. Thus, switching power supply has been widely used in such applications as personal computer or other situations that require light weight and small size.
Another application of the switching power supply is in a DC-to-DC environment. Often, DC battery power is supplied to an electronic device that is over and above what is actually required for the device to operate efficiently. In this case, the DC power is switched and stepped-down in amplitude to provide a voltage that is appropriate to operate the device. Since the duty cycle of the switching transistor can be varied over a wide range and the switching transistors draw very little current when operating, a DC output voltage can be produced to power an electronic device with minimal current drain. An example of a typical DC-to-DC power supply 100 is shown in prior art FIG. 1 where an oscillator 101 supplies a switching reference signal to a digital controller 103. The digital controller 103 provides control such as phase information to drive buffer 105. The buffer 105 provides a switching control voltage to a switching network 107 which also accepts an input voltage 108. The switching network 107 works to lower the input DC voltage to some predetermined DC level where it can pass through a filter 109. The output voltage and current can then be measured by a current limiter 111 which acts to control the amount of current available at output 110.
One problem associated with all switching power supplies is the noise or interference produced at the fundamental switching frequency as well as its harmonic frequencies. This can be very problematic if the switching supply is used with a radio receiver as these harmonics can easily extend into the very high frequency (VHF) and ultra-high frequency (UHF) radio spectrums. If the radio is attempting to receive radio signal at or near a harmonic of the switching supply, this can greatly interfere with reception and signal quality to the extent that the radio anticipates a signal being received while no legitimate RF signal is actually present. Thus, as seen in FIG. 1, one way to mitigate this type of noise is to change the switching frequency such that the noise produced at the fundamental and harmonic frequencies do not interfere with the desired receiving frequency. This is accomplished by altering the oscillator 101 to ultimately control the rate upon which the input voltage 109 is switched.
Thus, the need exists provide a DC-to-DC switching power supply where the switching frequency can be easily programmable in order to strategically select the switching frequency to prevent unwanted switching noise and interference in a radio receiver.